The present invention relates to the field of gas-turbine installations and more particularly it relates to axial-flow reversible turbines.
Most successfully the present invention can be employed in ship-building and in two-circuit turbojet aircraft engines.
The introduction of gas turbines into transport machine building and, particularly, into ship-building has brought about a critical problem of reversibility, i.e. the possibility of obtaining forward and reverse rotation in a single installation. The existing engineering solutions aimed at reversing the rotation of propellers, e.g. variable-pitch propellers, reversible hydraulic reduction gears, etc. are complicated, large and insufficiently maneuverable devices. These disadvantages become most conspicuous in gas-turbine installations since they are very high, compact and highly maneuverable units.
This has led to a natural consequence, i.e. to the use of a reversible turbine as the most obvious device capable of ensuring reversibility.
The steam turbines are provided with a separate counter-rotating turbine; however, the wheel rotors of the direct-rotating and counter-rotating turbines are located in separate casings which makes such a layout cumbersome and little adapted for use in gas turbine installations.
As a result, the attempts to solve the problem of reversibility went along the course of devising single-casing two-circuit turbines with two-section blades, each circuit being constituted by a direct-rotating or counter-rotating gas-flow duct with valve devices intended to deny the access of the working medium to the corresponding gas-flow duct.
Known in the previous art is an axial-flow reversible turbine manufactured by General Electric.
The casing of this General Electric turbine accommodates a nozzle assembly and a wheel rotor mounted successively in the direction of the flow of the working medium and provided with two-section blades. One section of the blades forms the duct for the flow of the working medium ensuring direct rotation of the turbine. The flow duct of the direct-rotating turbine incorporates a valve in the form of variable-incidence vanes intended to shut off the admission of the working medium into said duct. The variable-incidence vanes are arranged before the upper section rotor blades in the direction of the flow of the working medium.
The counter-rotating flow duct of the General Electric turbine accommodates variable-incidence vanes which deny the access of the working medium to the lower section of the counter-rotating blades. In this case the rotation axis of the counter-rotating blades coincides with that of the direct-rotating blades.
In the reversible turbine running at the direct-rotating mode the positions of the main turbine elements of this General Electric turbine are as follows:
(a) The variable-incidence vanes of the nozzle assembly in the gas flow duct of the direct-rotating turbine are in the "OPEN" or "TURBINE MODE" position and admit the working medium to the wheel rotor blades of the direct-rotating stage; at the same time the variable-incidence vanes of the nozzle assembly in the gas-flow duct of the counter-rotating stage are in the "CLOSED" position and the working medium is not admitted to the wheel rotor blades of the counter-rotating stage except for the leaks, i.e. that proportion of the entire flow which penetrates through the radial and peripheral clearances of the variable incidence vanes in the "CLOSED" position.
(b) Meanwhile the lower-section blades work in the "VENTILATION MODE" and consume a fraction of the work of the direct-rotating stage which is in the "TURBINE MODE". Meanwhile the direct-rotating turbine blades rotating with the edges forward consume a fraction of the work of the direct-rotating stage which is in the TURBINE mode. This fraction is referred to as "ventilation losses", while the corresponding turbine mode is a VENTILATION mode.
When the reversible turbine runs at the counter-rotating duty, the variable-incidence vanes of the nozzle assembly in the gasflow duct of the direct-rotating stage occupy the "CLOSED" position and deny the access of the working medium to the wheel rotor blades of the direct-rotating stage in which case the variable-incidence vanes of the nozzle assembly of the counter-rotating stage are in the "OPEN" position and the working medium is admitted to the wheel rotor blades of the counter-rotating stage. The wheel rotor blades of the direct-rotating stage are in the "VENTILATION MODE" and consume a fraction of the work done by the counter-rotating stage which is in the "TURBINE MODE".
In the single-stage embodiment of the General Electric reversible turbine the action of wide temperature differences calls for high tip speeds of the wheel rotors which raises sharply the ventilation losses between the tips of the blades and the surrounding casing of the counter-rotating turbine in spite of the fact that the outside diameter of the counter-rotating gas flow duct is substantially equal to the inside diameter of the direct-rotating gas flow duct. The ventilation losses are those losses which correspond to the work consumed and done by the counter-rotating turbine.
High tip speeds of the wheel rotors impose heavy stresses on the rotor blades. These stresses can be relieved by reducing the areas through the direct-rotating and counter-rotating gas flow ducts. This, in turn, will increase the exit velocities of the working medium which increase the pressure losses after the wheel rotors thus reducing the efficiency of the reversible turbine.
The General Electric reversible turbine has a low efficiency which is caused also by the fact that the inside diameter of the gas flow duct of the direct-rotating stage is substantially equal to the outside diameter of the gas flow duct of the counter-rotating stage, and since the modern gas-turbine engines are characterized by the need of withstanding big temperature differences or enthalpy higher than that withstandable in one stage of a high-efficiency turbine, these present considerable difficulty in developing a multi-stage direct-rotating turbine.
These difficulties arise because the provision of a multiple-stage direct-rotating turbine would call for the provision of an equal number of stages of a counter-rotating turbine and would result in an unduly sharp increase of ventilation losses which reduce the turbine efficiency.
In an alternative case it would be necessary, without increasing the number of stages of the counter-rotating turbine, to increase sharply the dimensions of its blades which is not rational either.
Besides, the efficiency of the known reversible turbine is also reduced because of a sudden expansion of the gas flow duct at the entrance of the working medium into the direction-rotating stage which leads to vortex generation and additional hydraulic losses. The sudden expansion of the gas flow duct is caused by the alignment of the valves in the form of variable-incidence airfoil vanes with the nozzle assemblies.
The variable nozzle assemblies are characterized by radial clearances which allow the working medium to leak into the counter-rotating gas flow duct with the turbine running at the direct-rotating duty; this reduces considerably the efficiency of the direct-rotating stage.
An object of the present invention is to provide a reversible gas turbine of a higher efficiency by introducing a smooth and pressure tight gas flow duct of the direct-rotating turbine and by creating a multiple-stage direct-rotating turbine ensuring the action of great temperature differences.
Another object of the present invention is to provide a reversible gas turbine with a higher specific power.